Hope is fading for finding survivors aboard a Russian-built Indian Navy submarine that exploded and sank in the port of Mumbai with 18 sailors aboard.

"While we hope for the best, at the same time we also have to prepare for the worst. The boat has remained submerged, all the compartments concerned, they have remained flooded for more than 12 hours now," Indian Navy chief Admiral D.K. Joshi told reporters on August 14.

Divers attempting to enter the submarine have been able to open one of the hatches of the submarine, which lies nearly submerged at its dock.

However, they have been unable to enter the vessel because it is dark and full of muddy water.

The cause of the blast is not immediately known.

"The basic question is what caused the fire and the explosions. We don't have an answer to the question as of now," Joshi said. "On any warship -- and it is more pronounced on a submarine -- there are several ingredients that can contribute to or cause a fire."

Several other people, including sailors stationed on vessels nearby, were reportedly taken to the hospital with injuries from the blast.

Navy spokesman P.V.S. Satish said the submarine was fully operational and was carrying a "full complement of torpedoes and missiles."

Reports say the 16-year-old, diesel-powered submarine had recently returned from Russia where it had been given an overhaul and upgrade.

There was an explosion aboard the same submarine in 2010 that killed one sailor and injured two others.

Russia has already offered to help in the inquiry into the explosion, noting there are currently eight specialists in Mumbai from the Zvezdochka center where the ship was overhauled.

Last year, India leased a Russian Nerpa nuclear submarine for 10 years at a cost of $1 billion. India has also built its own nuclear sub and its reactor was activated for the first time on August 10. That vessel is expected to go into service in the next two years.